In the case that a wing of an aircraft is made of a composite material including a carbon fiber or a glass fiber, a spar (beam member) having a U-shape in section is provided as a part of a structural member for the wing.
As illustrated in FIGS. 3A and 3B, a spar 1 formed into the U-shape in section, includes a web 2 and flanges 3 and 3 provided at both ends of the web 2.
In the case that the spar 1 is made of the composite material, sometimes a prepreg sheet in which a carbon fiber or a glass fiber is impregnated with a resin to form a sheet is used. In the case that the spar 1 is constructed by the prepreg sheet, the prepreg sheets are stacked to form plural layers. At this point, in each layer, a prepreg sheet 5 strip having a predetermined width is bonded such that a fiber direction of the carbon fiber or the glass fiber becomes a predetermined angle. In the prepreg sheets 5 that are vertically stacked next to one another, the prepreg sheets 5 are stacked such that the fiber direction of the prepreg sheet 5 varies. For example, when an axial direction of the spar 1 is set to 0°, the prepreg sheets 5 are bonded while a fiber direction of the prepreg sheet 5 is changed to 0°, 45°, 90°, . . . . Thus, strength of the spar 1 is enhanced in many directions.
In the case that the spar 1 is curved in the axial direction thereof in forming the spar 1, a wrinkle is easily generated in the prepreg sheet 5 in a ridge 4 that is of a boundary portion of the web 2 and the flange 3. Particularly, the generation of the wrinkle becomes prominent in the case that the angle of the bonding direction of the prepreg sheet 5 is as small as 0° with respect to the axial direction of the spar 1. This is because the prepreg sheet 5 has a poor stretch property in the fiber direction. In the case of the severe curvature, the wrinkle is easily generated in the prepreg sheet 5 in a portion in which the prepreg sheet 5 is three-dimensionally curved by the curvature in a direction in which the ridge 4 is continued in addition to the curvature by the web 2 and the flange 3 in a plane orthogonal to the ridge 4.
Conventionally, in the ridge 4 and a surrounding area thereof, a slit 6 is provided in a direction substantially orthogonal to the fiber direction in the belt-like prepreg sheet 5 when the prepreg sheets 5 are stacked. A gap between both sides of the slit 6 of the prepreg sheet 5 that is formed into a rectangular shape on both sides of the slit 6 is widened on an outer circumferential side in the curvature direction of the ridge 4. On an inner circumferential side in the curvature direction of the ridge 4, the prepreg sheet 5 portions on both the sides of the slit 6 are overlapped with each other, and the overlapped portion is removed (for example, see Patent Document 1).